Orthogonal multiple access, in which multiple signals do not interfere with each other, is widely used in communication between a base station and user equipments (e.g., mobile stations) in a mobile communication network. With orthogonal multiple access, different radio resources are allocated to different user equipments. CDMA (code division multiple access), TDMA (time division multiple access), and OFDMA (orthogonal frequency division multiple access) are examples of orthogonal multiple access. For example, in Long Term Evolution (LTE) standardized by the 3GPP, OFDMA is used in downlink communication. With OFDMA, different frequencies are allocated to different user equipments.
In recent years, non-orthogonal multiple access (NOMA) has been proposed as a method for communication between a base station and user equipments (e.g., see Patent Document 1). With non-orthogonal multiple access, the same radio resources are allocated to different user equipments. More specifically, a single frequency is allocated to different user equipments simultaneously. In applying non-orthogonal multiple access to downlink communication, a base station transmits a signal with a large transmission power to a user equipment (generally a user equipment at a cell area edge) with a large path loss, that is, a user equipment with a small reception SINR (signal-to-interference-plus-noise-power ratio), and the base station transmits a signal with a small transmission power to a user equipment (generally, a user equipment at the center of a cell area) with a small path loss, that is, a user equipment with a large reception SINR. Accordingly, the signal received by each user equipment is influenced by interference caused by signals addressed to other user equipments.
In this case, each user equipment demodulates the signal addressed to that user equipment using a power difference. Specifically, each user equipment first demodulates the signal with the highest reception power. Because this demodulated signal is a signal addressed to a user equipment that is the closest to the cell area edge (or more accurately, the user equipment with the lowest reception SINR), the user equipment closest to the cell area edge (the user equipment with the lowest reception SINR) ends demodulation. Each of the other user equipments cancels out the interference component, which amounts to that demodulated signal, in the received signals using interference cancellers, and demodulates the signal with the second-highest reception power. Because this demodulated signal is the signal addressed to a user equipment that is the second-closest to the cell area edge (or more accurately, the user equipment with the second-lowest reception SINR), the user equipment that is the second-closest to the cell area edge (has the second-lowest reception SINR) ends modulation. By thus repeating the demodulation and canceling out of signals with high power, all of the user equipments can demodulate the signals addressed to them.
By combining non-orthogonal multiple access with orthogonal multiple access, it is possible to increase the capacity of the mobile communication network in comparison to using orthogonal multiple access alone. That is, in the case of using orthogonal multiple access alone, it is not possible to allocate a certain radio resource (e.g., a frequency) to multiple user equipments for the same duration, but in the case of combining non-orthogonal multiple access and orthogonal multiple access, a certain radio resource can be allocated to multiple user equipments for the same duration.
The following three interference cancellers are representative candidates to be used in NOMA (Non-Patent Document 1).
Symbol-level Interference Canceller (SLIC)
This handles interference signals at the symbol level (i.e., for each RE (resource element)) and cancels out the demodulation result of the interference signal.
Codeword-level IC (CWIC)
This is also referred to as a Turbo SIC (Successive Interference Canceller) or Codeword SIC, decodes the interference signal at the codeword level and cancels out the decoding result. For example, Non-Patent Document 2 discloses a Codeword SIC.
Maximum Likelihood (ML)
This jointly estimates desired signals and the interference signals at the symbol level (i.e., for each RE (resource element)).
In order to improve the performance of NOMA, a receiver having a highly-accurate interference canceller is desirable, and therefore application of CWIC is desirable. However, in order to improve the accuracy of the interference canceller, the amount of required information on the interference signal increases. With CWIC, the result of decoding the interference signal is canceled out, and therefore there are more types of required information elements for the interference signal than with other interference cancellers. Section 7.5 of Non-Patent Document 1 discloses information that is needed in CWIC. Also, since other interference cancellers also cancel out the result of demodulating an interference signal, various types of information are needed in order to demodulate the interference signal.
Here, an interference signal is a data signal that gives interference to the desired data signal of a user equipment, with the data signal giving interference being addressed to another user equipment. In LTE, demodulation or decoding of a data signal requires information included in a control signal corresponding to a user equipment that is the destination of that data signal. Accordingly, the interference canceller needs to decrypt a control signal corresponding to another user equipment.
Patent Document 1 discloses various methods according to which a mobile station recognizes control information of another mobile station in a radio communication system using non-orthogonal multiple access.